The present invention relates generally to the field of scanning electron microscopes used as test probes for visualizing and testing integrated circuits and more particularly to an improved apparatus and method for using said test probes in a stroboscopic mode.
As a result of the progress in the design and fabrication of integrated circuits, it has become possible to create circuits having millions of conductors and transistors in which the individual conductors and nodes are of the order of one to two microns. These circuits are too small and complex to be amenable to testing and analysis by techniques using mechanical probes. The mechanical probes tend to capacitively load the circuits under test thus altering the behavior one wishes to measure. Further, the mechanical probes may actually physically damage the minute conductors and nodes with which they come in contact. Finally, the number of nodes which must be examined to debug a VLSI integrated circuit is rapidly becoming too large to be amenable to manual measurement one node at a time. As a result, test probes based on electron beams have been developed. These test probes provide a means for measuring the potential on minute conductors as well as a means for forming an image of the conductors and the surrounding circuitry without any physical damage thereto.
Such an electron beam test probe is described in the above mentioned co-pending application which is hereby incorporated by reference. In general, electron beam test probe systems measure the potential at a specified point on the surface of the integrated circuit by sensing the energy distribution of the secondary electrons produced when the point in question is bombarded by electrons. The electron beam test probe system includes a means for generating an electron beam which may be directed at any point within a specified region on the integrated circuit surface. The interaction of this electron beam with the surface of the integrated circuit results in the production of secondary electrons whose energy distribution is related to the potential on the surface of the integrated circuit at the point in question. These secondary electrons are collected and the fraction of them with energies greater than a predetermined energy is determined by detecting the number of secondary electrons which have sufficient energy to overcome a potential barrier and reach an electron detector.
Since a finite time interval is required to collect sufficient secondary electrons to provide a statistically significant measurement of the potential on the surface of the integrated circuit, the potential actually measured is the average of potential over the time interval in question. This time interval is often too long to accurately measure the rapidly changing potentials which are present when a rapidly changing test signal pattern is applied to the integrated circuit under test. To avoid this problem, the electron beam test probe must be run in a stroboscopic mode which requires that the electron beam be turned on for a very short time interval at a precise time relative to the start of each cycle of a repetitive test signal pattern.
The prior art apparatus for pulsing the electron beam consists typically of a beam aperture and a pair of blanking electrodes. The electron beam must pass through the beam aperture to reach the integrated circuit being tested. When a potential is applied to the blanking electrodes, the electron beam is deflected in a manner which causes it to miss the aperture. The prior art blanking electrodes are typically a pair of deflection plates between which the electron beam passes before reaching the aperture.
The shortest time in which the electron beam may be turned on and off depends on the rise time of the potential applied to the blanking electrodes, the magnitude of said potential, and the length and separation of the blanking electrodes. The magnitude of the potential which must be applied to the blanking electrodes to sufficiently deflect the electron beam so that it will miss the aperture depends on the separation and length of the blanking electrodes. In general, large potentials are to be avoided, since it is more difficult to produce a large potential change with a short rise time. Hence, one must either use long blanking electrodes or place them close together. However, long blanking electrodes are also to be avoided, since the minimum time in which the electron beam may be turned on and off is proportional to the length of the blanking electrodes. Consequently, prior art systems are forced to use very small blanking electrodes which are separated by a very small distance. This results in significant problems in both aligning the blanking electrodes relative to the electron beam and in mounting the blanking electrodes on the apparatus in which the electron beam is generated.
In addition, the blanking electrodes have a finite capacitance which must be driven by the circuit which provides the blanking potential. At the high electron beam stroboscopic frequencies needed to analyze modern integrated circuits, this parasitic capacitance makes it difficult to impedance match the blanking electrodes to the signal generator used to supply the blanking potential.
Finally, the short duration of the electron beam pulse needed to analyze circuits running at high frequencies makes the generation of the blanking potential difficult. Electron beam pulses having durations of less than 100 picoseconds are often needed. It is difficult to generate blanking potential pules with rise times less than 100 picoseconds. Prior art systems have attempted to solve this problem by sweeping the electron beam across a small aperture such that the size of the aperture and the sweep speed determine the duration of the electron beam pulse. Although this type of system allows a blanking potential with a much slower rise time to be used, it requires a second set of deflection electrodes which are used to prevent the electron beam from sweeping through the aperture as the electron beam returns to its original position in preparation for the next electron beam pulse.
Broadly, it is an object of the present invention to provide an improved blanking electrode system for use in electron beam test probe systems.
It is another object of the present invention to provide a blanking electrode system which significantly reduces the alignment problems inherent in prior art blanking electrode systems.
It is a further object of the present invention to provide blanking electrodes which may be easily matched to the impedance of the signal generating circuit used to drive said blanking electrodes.
It is a still further object of the present invention to provide a blanking electrode system which can produce very short duration electron beam pulses without requiring a second set of deflection electrodes.
These and other objects of the present invention will become apparent from the following detailed description of the present invention invention and the accompanying drawings.